Pregnancy week calculation: how to know when you will give birth
Often when you meet a pregnant woman you ask her what month of pregnancy she is. However, it would be much more appropriate to ask the expectant woman her week of pregnancy for two specific reasons: the first is that the fetus develops rapidly from day to day and from week to week it makes incredible progress and changes. The second reason is that the calculation of the week of pregnancy does not correspond exactly to the whole month from conception.
The months are thus divided in a different way from the conventional ones: in order to make an accurate calculation of the expected date of delivery, it will be necessary to count 280 days from the beginning of the last menstruation. But, as we know, the human body is not a perfect machine, and for this reason the DPP (estimated date of birth) is completely relative: in fact, it is necessary to consider a much longer period of time, that is, from the 38th to the 42nd week.
Calculation of the expected date of birth
In order to calculate the week of pregnancy, it is necessary to follow a rule, the Naegele rule, in which, in order to arrive at the expected date of birth, seven days are added and three months are subtracted from the date of the first day of the last cycle. So if, for example, today (20/07/2021) is the first day of the last cycle, the expected date of birth will be 27/04/2022.
In order to calculate the weeks of pregnancy, gynaecologists usually provide expectant mothers with a very small card where the months of pregnancy are divided (obstetrical rule or wheel of pregnancy). This is because in order to calculate the number of weeks of pregnancy it is not enough to take into consideration the expected date of birth and add nine months (because the months are not composed of precisely 4 weeks), but it will be necessary to do some more complex calculations. Here is the scheme to calculate the weeks of pregnancy and to know the respective month:
- First month: from the first day of the last menstruation to 4 weeks 3 days
- Second month: from 4 weeks 4 days to 8 weeks 5 days
- Third month: from 8 weeks 6 days to 13 weeks 1 day
- Fourth month: from 13 weeks 2 days to 17 weeks 4 days
- Fifth month: from 17 weeks 5 days to 21 weeks 6 days
- Sixth month: from 22 weeks 0 days to 26 weeks 2 days
- Seventh month: from 26 weeks 3 days to 30 weeks 4 days
- Eighth month: from 30 weeks 5 days to 35 weeks 0 days
- Ninth month: from 35 weeks 1 day to 40 weeks 0 days
When does a pregnancy start and end
Conventionally, the beginning of pregnancy is set at the first day of the last menstruation, because it is very difficult to know the date of ovulation (approximately two weeks after the last menstruation) and conception.
This is because the DPP can vary according to many factors such as the date of conception, the duration of the future mother's menstrual cycles and the duration of the pregnancy in which the birth can be anticipated or delayed.
A physiological pregnancy, in fact, reaches its term with tranquillity even between 37 and 42 weeks, therefore the presumed date of birth can be taken as purely indicative.
Anamnestic DPP and ultrasound DPP
In order to calculate the weeks of pregnancy it is necessary to make another distinction, that is, between anamnestic and ultrasound DPP.
In fact, we have said that the calculation of the weeks begins with the first day of the last cycle, but the gynaecologist, on the basis of the first ultrasound scan of the foetus, could establish two dates:
- the anamnestic expected date of delivery
- the ultrasound estimated date of delivery
The anamnestic expected date of delivery is determined by Naegele's calculation, while the ultrasound expected date of delivery is determined by the parameters of the fetus determined by the fetal ultrasound, based on its fetal development.
If the two dates correspond, the pregnant woman will certainly have a regular cycle and her ovulation will take place exactly on the 14th day of the cycle, vice versa, if the dates do not correspond, the woman will certainly have an irregular cycle or a not very detailed monitoring of her menstrual cycles.
The DPP is also useful to indicate the care plan of the future mother: a premature baby will certainly need more tests and more care, while for a too long gestation it could be decided to induce the birth by means of oxytocin or a caesarean section.
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